Test Methods and Effect on Properties of Soil

Soil compaction is a fundamental engineering process that involves the mechanical compression of soil particles, reducing the void spaces between them while simultaneously increasing the mass density.

Compaction of Soil

This crucial procedure is indispensable for various construction projects, such as earth dams, canal embankments, highways, runways, and many other structures. This article explores the methods of testing soil compaction, the factors influencing it, its effects on soil properties, and the various techniques employed in the field.

Methods of Testing Compaction of Soil

Standard Proctor’s┬áTest for Compaction of Soil

The Standard Proctor’s test is a laboratory-based approach to determine the water content and dry density relationship, providing essential insights for field compaction. It employs a standard mold with a 4-inch internal diameter and an effective height of 4.6 inches, accommodating 1/30 cubic foot of soil. Compaction is carried out in three layers, each subjected to 25 blows from a 5.5-pound rammer, falling through a height of 12 inches.

IS: 2720 part VII adheres to similar specifications, with minor modifications. It recommends a 100mm diameter mold, 127.3mm height, and 1000 ml capacity. The rammer should weigh 2.6 kg with a free drop of 310mm and a face diameter of 50mm. Compaction is again performed in three layers, with a 60mm collar attached to the mold.

Procedure for Standard Proctor’s Test:

  • Approximately 3kg of air-dried soil is taken and mixed with 8% water content.
  • The mixture is filled in the mold in three layers, with each layer receiving 25 blows.
  • The volume and mass of the compacted soil are measured, and bulk density is calculated.
  • A sample is placed in the oven to determine water content.
  • Dry density is calculated from the bulk density and water content.
  • This process is repeated while incrementally increasing the water content.

Presentation of Results

  • A compaction curve is plotted, showcasing the relationship between water content and corresponding dry density.
  • The curve demonstrates that dry density initially increases with water content until it reaches the maximum density.
  • Beyond this point, an increase in water content reduces dry density, as excess water occupies space meant for solid particles.
  • The water content corresponding to the maximum dry density is termed the Optimum Water Content (O.W.C) or Optimum Moisture Content (O.M.C).
  • Theoretical maximum density is achieved when the soil is 100% saturated, and this value is referred to as saturated dry density, with the associated line termed the zero air void line.

Modified Proctor Test for Compaction of Soil

The Modified Proctor test simulates heavier compaction conditions than the Standard Proctor test and is designed to replicate scenarios where heavy rollers are employed. It conforms to the standards set by the American Association of State Highway Officials (AASHO).

The mold used in the Modified Proctor test is identical to the Standard Proctor test. However, the rammer is considerably heavier, weighing 4.89 kg, with a free drop of 450mm. Compaction is achieved in five equal layers, each subjected to 25 blows, resulting in a competitive effort 4.56 times greater than that of the Standard Proctor test.

  1. Water Content: Soil compaction is influenced by water content, with low water content yielding stiff soil that resists compaction. As water content increases, soil particles become more workable and pack more closely, leading to higher dry density until reaching the O.M.C.
  2. Amount of Compaction: Increased compaction effort enhances dry density at lower water content levels up to a certain point.
  3. Type of Soil: Different types of soil exhibit varying O.M.C and dry density characteristics.
  4. Method of Compaction: The chosen compaction method significantly impacts the achieved dry density.

Effect of Compaction on Properties of Soil

Soil compaction affects various properties, including:

  1. Soil Structure: Soil compacted below the O.M.C typically exhibits a flocculated structure, while compaction above the O.M.C results in a dispersed structure.
  2. Permeability: Soil permeability decreases with increasing water content on the dry side of the O.M.C due to changes in void sizes.
  3. Swelling, Pore Water Pressure, Shrinkage, Compressibility, Stress-Strain Relationship, and Shear Strength are all influenced by soil compaction.

Methods of Compaction of Soil Used in the Field

Several methods are employed in the field for soil compaction, with the choice determined by factors such as soil type, required maximum dry density, and economic considerations. Common methods include:

  1. Tampers
  2. Rollers
  3. Vibratory compactors

The effectiveness of compaction depends on various factors, including contact pressure, the number of passes, layer thickness, speed of the roller, and roller type (e.g., smooth wheel, pneumatic-tired, or sheep foot rollers).

Controlling Compaction of Soil

Compaction control in the field is achieved through measurements of dry density and water content. Dry density is determined using methods such as the core cutter or sand replacement method. Water content is measured using techniques like the oven drying method, sand bath method, calcium carbide method, and the Proctor needle.

Soil compaction plays a pivotal role in civil engineering, ensuring the stability and durability of various structures. Understanding the testing methods, factors influencing compaction, its effects on soil properties, and the methods used in the field is essential for successful construction projects and optimal engineering performance.

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